Martha: Thing is, though, am I missing something here? The world didn't end in 1599, it just didn't. Look at me, I'm living proof.
The Doctor: Oh, how to explain the mechanics of the infinite temporal flux? I know, Back to the Future. It's like Back to the Future.
Martha: The film?
The Doctor: No, the novelisation. Yes the film! Marty McFly goes back and changes history.
Martha: And he starts fading away... Oh my God, am I going to fade?
The Doctor: You and the entire future of the human race. It ends right now in 1599 if we don't stop it.